Swiss authorities have frozen $550m in bank accounts belonging to the late Nigerian dictator, Sani Abacha, members of his family and his associates.

The order covers about 120 accounts in a dozen banks based in Zurich and Geneva.

The chief prosecutor in Geneva, Bernard Bertrossa, said a money-laundering inquiry had been opened.

This investigation could result in the millions being returned to Nigeria.

But our Geneva correspondent, Claire Doole, says the accounts could be unfrozen in a month's time if Nigeria fails to present a formal request for legal assistance.

To do that, the Nigerian authorities must show a specific link between their domestic investigations and the blocked assets.

General Abacha, his family and associates are accused of systematically plundering Nigeria's central bank during his four years in power.

The accounts under scrutiny belong to the general himself, to his former security advisor Alhaji Ismaila Gwarzo, former government minister Abubakar Attiku Bagudu, as well as a number of unidentified Nigerian businessmen and companies.

The military leader died suddenly in 1998, and political reforms initiated by his successor led to the restoration of democratic rule this year.

Money laundering

Swiss officials first announced the freezing of the Abacha accounts in October, but this is the first time any specific sum has been mentioned.

Switzerland has often been criticised as a money-laundering centre, with several former foreign rulers currently under investigation.

A law which came into effect last year obliges banks to report and freeze suspicious accounts.

But Prosecutor Bertrossa said that the discovery of the Abacha millions highlighted problems which certain Swiss banks had in complying with the new legislation.

The Swiss bank accounts of the late President of Zaire, Mobutu Sese Seko, are still frozen pending legal action.